If I run perf top with a "make -j128" kernel build, I get ring buffer event processing timeouts which results in:
ui__warning("Too slow to read ring buffer.\n" "Please try increasing the period (-c) or\n" "decreasing the freq (-F) or\n" "limiting the number of CPUs (-C)\n"); from perf_top__mmap_read(). This hangs the main event thread. Only the display thread runs after this point. We can't issue UI messages from the event thread, because those will hang waiting for a keypress. The display thread will eat any keys we press and the event thread thus hangs forever. I can tell this is what has happened because the histogram entries continue to decay, yet the event count stops increasing. If I put a gdb on the perf process, indeed the backtrace in the event processing thread is in the select() call done by ui__getch(). Adding insult to injury, the display thread immediately overwrites the warning message printed by the event thread, and thus the user has no chance to even see it. I really wonder how this was tested. Perhaps we should mark the event thread in a special way and trigger assertions if UI messages are printed from it. Again, any such operation will hang the thread and stop all event processing.